This invention relates to a large-depth underground drainage facility which is arranged such that influent flow, such as rainwater and the like flowing into water channels including small rivers, is collected in an inflow channel provided underground, and the collected influent flow is led to a pump station and then is discharged therefrom to a river or the like. More specifically, the invention relates to such a large-depth underground drainage facility which can be built at less costs, and also to a method of running such an underground drainage facility.
There is known an open channel flow operation in which drain water from flood control channels, provided in and near to the ground surface, flows through vertical shafts, and then is led to a pump station through an underground water channel, provided in an underground of a large depth, in such a manner that a space is maintained in the underground channel above a water level of this underground channel (that is, the underground channel is not fully filled with the drain water). There is also known a closed channel flow operation in which the drain water flows through the underground channel in such a manner that the drain water fully fills the underground channel.
In the open channel flow operation, in order to minimize the risk that the water overflows from the vertical shafts to the ground surface when the drain water flows abruptly into the underground channel, the underground channel has a large capacity, that is, a diameter, for example, of 12.5 m, so that the water level in the underground channel can be kept low.
The level of an impeller of a pump provided at a downstream end of the underground channel needs to be below the level of draining of the underground channel so that the underground channel can always be made empty. Generally, the lowest water level L.W.L is disposed near to the bottom of the underground channel. The pump is provided at the square pump station, and the pump is started and stopped in accordance with predetermined levels of the water in a pump well.
On the other hand, in the closed channel flow operation, the pump is operated when at least the underground channel is filled with the water and further the water level rises into the vertical shafts. This condition is encountered when the amount of influent flow exceeds the draining ability of the pump.
For estimating the rate of flow into the pump station, the amount of rainfall is estimated in accordance with rainfall information, and the amount of flow into each vertical shaft is estimated in accordance with an outflow factor representative of the ratio of this rainfall amount to the amount of rainwater flowing into rivers. The amounts of flow into all the vertical shafts are totaled to determine the estimated rate of flow into the pump station.
Since the pump of the pump station for carrying out the conventional open channel flow operation is disposed at the level near to the bottom of the underground channel, the head (lift) Ha between the water level of a discharge water tank and the lowest water level L.W.L is large, and therefore the total head of the pump is large, and the installation cost of the pump station including pumps and a drive machine is high. And besides, since the water level of the underground channel is kept low, a storage effect is low, and the water level drops soon to stop the pump, and then the water level is recovered in a short time, so that the pump is again started, which often results in hunting.
Since the underground channel having a length of several kilometers has a large diameter, the cost of excavating the underground channel, which accounts for a larger proportion of the installation cost of the large-depth underground drainage facility, is high. Since the pump is to be installed in an underground of a large depth, the excavation cost of the pump station is also high.
On the other hand, in the closed channel flow operation, the water level of the underground channel is higher as compared with the open channel flow operation, and therefore the total head of the pump is small, and the installation cost of the pump station including the pumps and the drive machine is lower; however, there is encountered a higher possibility that the water overflows from the vertical shafts to the ground surface.
With respect to the conventional outflow factor for estimating the inflow amount, no consideration has been given to the fact that when the interval between rainfalls is short, the rainwater hardly penetrates into the ground, but flows into the drainage facility to increase the inflow amount. Thus, the accuracy of the estimated amount of flow into the vertical shafts is low with respect to a change in the interval between rainfalls. This may result in a possibility that the pump station is not properly operated, so that the water may overflow from the vertical shafts.